The effective protection of structures such as buildings, conventional and nuclear power plants, factories and the like against the effects of earthquakes remains an imperfectly attained goal. When an earthquake strikes, normally along an earthquake fault, seismic shock waves (meaning those caused by the earthquake) of three types emanate from the hypocenter of the earthquake, the initial point of rupture along the fault typically several kilometers or more below the surface. Of these seismic waves, shear waves, also called S (for secondary) waves, generally cause the greatest damage to structures. Compressional waves, also called P (for primary) waves, normally cause less damage. Surface waves, which are generated by the P and S waves when they reach the surface, travel along the ground surface and may also cause damage.
It is known that buildings can be protected from vibrations caused by blasting, traffic and the like by digging a trench a few feet deep between the blasting site and the building and filling the trench with a suitable material to thereby form a wall. U.S. Pat. No. 1,728,736 discloses the use of a subterranean wall placed adjacent the foundation of a house, or between a roadway and a sidewalk abutting the foundation, to protect the house from vibrations caused by vehicles on the roadway. The wall, which is about 6 feet deep, is made of a resilient material such as rubber or materials such as compressed cork, asbestos, bricks, or reinforced concrete blocks coated with rubber. These walls are relatively shallow barriers which may be effective against blasting and roadway vibrations because the wave lengths emanating therefrom are short, typically in the order of a few cm. The vibrations, having relatively short wavelengths and correspondingly short amplitudes, cannot merely bypass the barrier even though it is relatively shallow. However, such vibration isolation methods are ineffective protections against seismic waves because these waves have lengths of as much as 1/2 km or more. Therefore these relatively shallow trenches or walls merely move as a part of the earth's surface during an earthquake and provide no seismic protection whatsoever. In addition, the walls of the type disclosed in the referenced patent readily transmit S waves irrespective of the depth of the walls because of the substantial shear moduli of the materials from which they are made. Consequently, they cannot provide protection against seismic waves.
In the past, various attempts have been made to protect buildings against the effects of earthquakes by partially decoupling the foundation from the ground. U.S. Pat. No. 4,166,344 shows a building supported by slidable pads and held in place by a number of frangible links between the structure and the ground. When the horizontal motion of the ground exceeds a predetermined magnitude, the frangible links break allowing the ground to move beneath the building, thus reducing the amount of force transmitted from the ground to the building. U.S. Pat. No. 3,748,800 discloses an earthquake isolation foundation wherein the building is supported on a spring-centered building base in a water filled excavation. This structure is intended to reduce earthquake-induced accelerations, horizontally and vertically, on the building. Alternatively, the building base can be supported on sand, rather than on water, to provide a degree of isolation from horizontal movement of the ground.
However, none of these prior art methods can protect an existing building, such as a nuclear reactor and associated critical auxiliary structures, from the effects of an earthquake without extensive modification to at least the building's foundation. Depending upon the specific building and the character of the site, modification of the foundation according to the teaching of the prior art may be technically and/or economically unfeasible. Further, with the prior art protection methods the structural design of the building must be compatible with the protection method chosen. For some structures, such as nuclear power plants, design considerations may make the prior art methods inappropriate for incorporation into the design of the building.
Thus, what is lacking in the prior art is an effective system to protect both existing and future buildings against earthquake damage.